The present invention is directed to controlling temperature in a cryogenic cooling operation that disperses a cryogen flow, and in particular, it is directed to a dual phase cooling system that provides variable temperature control using a proportional mixture of cryogen liquid and gas, the gas fraction controlled by a proportional valve modulated in response to temperature feedback from the cryogen flow.
Controlled cooling is critical in various manufacturing and operating environments, for example, during machining and/or rolling a product, thermal spraying, de-molding, quenching, and other like operations. Precise temperature control of the dispersed cryogen coolant, and at times, temperature control along tool surfaces, is required in many machining and rolling operations. Using a cryogen coolant in such applications has limited applicability due to the likelihood of overcooling or undercooling of the target product surface for any of the following reasons: (a) the heat generated during machining or rolling is smaller or larger than the heat capacity of the delivered coolant, or (b) irreversible changes in the product bulk or surface properties occur at the extremely cold temperatures generated by cryogens. Wide temperature fluctuations are detrimental to materials that are susceptible to cold temperatures causing surface damage and/or material fractures in the product.
Past attempts to provide controlled cryogenic cooling temperature at the target surface or contiguous atmosphere involve varying cryogen flow rates so that more or less cryogen is delivered to the target surface. This is accomplished by using different nozzle orifice sizes, different size restrictions in the cryogen feed lines, or periodic cycling of a cryogenic solenoid that modulates the coolant flow rate. In such instances, the cryogen flow rate is matched with heat generated within the operating environment.
Varying cryogenic temperatures with heaters has also been used in the past to provide temperature control. However, such processes do not include temperature feedback and are not suited for variable thermal loads. The use of external heaters is difficult and cumbersome to implement, and their use provides inaccurate or approximate temperature control that results in wide temperature fluctuations. The use of internal heaters is also problematic, due to slow reaction time and the setup and control difficulties associated with a heater located in a bath of liquid cryogen and within a cryogenic fixture.
Attempts to adapt non-cryogenic temperature feedback systems for use with cryogenic coolants have been unsuccessful for several reasons. Precise temperature control of low boiling point cryogen coolants at subzero temperatures is extremely difficult due to significant heat loss to the environment. When cryogenic liquids are delivered at low flow rates, heat loss (or the heat sink effect) is extremely critical. In addition, cryogenic liquids constantly boil off into large volumes of cryogenic gases. One volume of liquid nitrogen (LIN) transforms into 693 volumes of nitrogen gas (GAN) at room temperature. Large volumes of gas can impeded cryogenic liquid flow, particularly when the cryogenic fluid is transported over long distances, making temperature control extremely difficult. Pressure drops often occur during transport of liquid cryogens over long distances. With any sudden pressure drop, a portion of the cryogenic liquid is transformed into gas that causes fluctuations in the flow rate if not removed from the feed line. Also, due to the difficulty of using adjustable valves on a cryogenic fixture, providing real-time adjustment of the delivery pressure of cryogen coolants is problematic.
Related prior art includes U.S. Pat. Nos. 4,484,457, 4,848,093, 5,647,228 and 6,513,336.